The present invention relates to a blow molding method and apparatus in which a parison composed of a hot plastic material is inflated within a mold with a pressurized blowing gas to form an article and thereafter the article is cooled by introducing a cryogenic cooling fluid into the article. More particularly, the present invention relates to such a method and apparatus in which the cryogenic cooling fluid and optionally the blowing gas is formed by combining liquid and gaseous forms of a cryogen and recovering the warmed mixture from the article in order to supply the gaseous form of the cryogen.
In the production of blow-molded articles by a variety of techniques (notably extrusion blow molding where a hot parison formed of a plastic material is blown within a mold), a significant portion of the time required to mold the article is spent in cooling the article. Since the total time required to mold each article will effect production capacity, a reduction in the time spent in cooling the article will increase production capacity. The prior art has thus supplied various apparatus and methodology involved in the reduction of cooling time. Such apparatus and methodology has involved using a cryogenic gas or liquid as a cryogenic cooling fluid for cooling the article.
For example, U.S. Pat. No. 3,505,422 provides a method and apparatus for cooling a blow molded article in which a cooling fluid, such as carbon dioxide at as low a temperature as -25.degree. F. is injected into the article, once molded. After removal of the article from the mold, the carbon dioxide is allowed to escape from the article. In order to prevent such wastage of the cooling fluid, U.S. Pat. No. 3,065,501 provides a method and apparatus in which the blow molded article is cooled with liquid carbon dioxide, and the resultant carbon dioxide vapor produced from the vaporization of the liquid carbon dioxide is reclaimed and reliquefied for use in subsequently cooling the article within the mold. U.S. Pat. No. 4,367,187 provides yet another method and apparatus for cooling a blow molded article. In this patent, after the article is partially blown, liquid nitrogen is injected into the article. This causes the liquid nitrogen to expand into a gas to complete the blowing of the article. After the article is blown, the nitrogen gas is reclaimed and stored for use in partially blowing an article to be subsequently formed within the mold. The disadvantage of these last two mentioned patents relates to their use of a liquid cryogen as a cooling fluid. When liquid cryogen is used in this manner, the cooling of the article may not be uniform throughout the article.
Methods and apparatus for utilizing liquid carbon dioxide are disclosed in U.S. Pat. No. 3,520,020, U.S. Pat. No. 3,450,805 and U.S. Pat. No. 3,666,849. In accordance with these last mentioned patents, liquid carbon dioxide is mixed with air to form a gaseous cooling fluid that is used in cooling the blow molded article. The advantage of such a gaseous cooling fluid is that the blow molded article can be more uniformly cooled in the mold as compared with the use of the pure liquid cryogen. Additionally, since the mixture of air and vaporized liquid cryogen is being injected into the molded article, none of the cooling potential of the liquid is lost, such as would be the case if the liquid cryogen were vaporized with an exterior heat source. The drawback of the process and methodology is that air contains moisture, and as such, expensive air dryers typically must be used to dry the air sufficiently so that ice formation and build up is prevented.
As will be discussed, the present invention provides a method and apparatus in which the blow molded articles are cooled with a cryogenic cooling fluid in the form of a gas or a cryogenic mist carried within a cryogenic gas in order to produce more uniform cooling of the article. As will be further discussed, the present invention has the desirable feature of utilizing the full cooling potential of the liquid cryogen. However, since air is not used to vaporize a liquid cryogen, cooling temperatures can be achieved that are as low as the liquid cryogen without requiring the use of air dryers.